An extremely important concern for applications of the recently discovered high temperature superconductors (HTS) has been the development of repeatable, reasonably high quality hysteretic and non-hysteretic Josephson junctions.
Non-hysteretic Josephson junctions have been demonstrated in HTS materials by a variety of microelectronic-compatible techniques. Most of the process development to date for these non-hysteretic Josephson junctions has involved engineering YBaCuO, as grown, grain boundary junctions into circuits located on the substrate in the layout; however, one alternate technique offers HTS junctions with at least a quasi-integratable process. The process involves artificially etching a step-edge into a substrate before YBCO film growth, as set forth in Daly, K. P. et al., Appl. Phys. Lett., Vol. 58, p. 543 (1991). If the step-edge angle is sufficiently steep and the step height to film thickness ratio is reasonable, grain boundary junctions form with good uniformity and yield. Epi-layer induced YBCO junctions are described in Char, K. et al., Appl. Phys. Lett., Vol. 59, p. 733 (1991). While not offering the full spectrum of applications of traditional tunnel junction technology, these devices have many potential uses including SQUIDs, Single Flux Quantum (SFQ) logic, shock-wave lines, and long-junction amplifiers.
Early Tl junction attempts relied on finding naturally occurring grain boundaries which is not reasonable for microelectronic applications. Indeed, other TlCaBaCuO junctions have relied on native grain boundaries, see e.g., Miklich, A. H. et al., Appl. Phys. Lett., Vol. 59, p. 742 (1991). These junctions lack the microelectronic-like nature of the inventive technique described herein; they are not as uniform, have a lower yield, and have lower figures of merit.
The grain boundary chemistry is very different in the various HTS materials such as YBaCuO (YBCO), BiSrCaCuO and related compounds (Bi), and TlCaBaCuO (Tl). The grain boundaries in the YBCO system tend to be loosely coupled and normal-conducting in nature. In poorly deposited films, this is the most common cause of poor electrical performance. In the Bi and Tl systems, the grain boundaries tend to be more insulating in nature possibly because the structure causes greater oxygen excesses in the boundaries. The Bi and Tl grain boundaries also tend to be more strongly linked in bulk material and superconducting property degradation in high magnetic is more the result of the material itself, rather than the grain boundaries, as with YBCO.
Because of the insulating nature of the boundaries of Tl materials, they are attractive for high performance junctions where a good insulating barrier is desired. If the grain boundaries can be weakened without destroying their material properties, successful high performance devices are possible. The growth of HTS material on the substrate step may be sufficient to weaken the boundaries and should not alter the fundamental boundary structure significantly.
The following documents are incorporated by reference: Morphology Control and High Critical Currants in Superconducting Thin Films in the Tl-Ca-Ba-Cu-O System, Ginley, D. S., Kwak, J. F., Venturini, E. L., Morosin, B. and Baughman, R. J., Physica C 160, 42 (1989), Fabrication of TlCaBaCuO Step-Edge Josephson Junctions with Hysteretic Behavior, Martens, J. S., Hietala, V. M., Zipperian, T. E., Vawter, G. A., Ginley, D. S., Tigges, C. P., Plut, T. A. and Hohenwarter, G. K. G., Appl. Phys. Lett., Vol 60, No. 8, pp 1013-1015, Feb. 24, 1992, and TlCaBaCuO Step-Edge Josephson Junctions, Martens, J. S., Zipperian, T. E., Vawter, G. A., Ginley, D. S., Hietala, V. M., and Tigges, C. P., Appl. Phys. Lett., Vol 60, No. 9, pp. 1141-1143, Mar. 2, 1992.
For non-hysteretic Josephson junctions, we have invented a step-edge process to produce repeatable, high quality Josephson junctions using the HTS materials of the TlCaBaCuO system. The Tl system is desirable because of its higher T.sub.c (up to 125K) compared to the more common YBa.sub.2 Cu.sub.3 O.sub.7 system with its lower T.sub.c (92K), and its large value of the product I.sub.c R.sub.n incurred while operating at a temperature of 77K which is indicative of high junction quality. Testing of over 250 junctions produced using the subject process has resulted in a yield of over 70% at temperatures to 100K.
One way to create hysteretic Josephson junctions from available non-hysteretic high temperature superconducting junctions is to artificially add capacitance. Hysteretic HTS junctions open up the possibility for many circuits which operate above a temperature of 77K. Hysteretic junctions are needed for voltage-state latching logic, SIS mixers, and studies of the superconductor gap structure of HTS materials. In addition, the coupling of very fast Josephson junction signals to the nonsuperconducting world becomes simpler with hysteretic junctions.
It is thus an object of the invention to provide a HTS Josephson junction that operates at higher temperatures, is compatible with microelectronic techniques, has a high degree of quality, a high yield and good uniformity.
Another object of the invention is to provide a HTS Josephson junction with hysteretic properties through the addition of an artificial capacitor to the non-hysteretic Josephson junction.